Typical photographic elements use silver halide emulsions, the silver halide having a native sensitivity to ultraviolet UV radiation ("UV"). Such UV sensitivity is usually undesirable in that it produces an image on the photographic element which is not visible to the human eye. In addition, in the case of color photographic elements, in particular, color dye images formed on the light sensitive emulsion layers by color development easily undergo fading or discoloration due to the action of UV. Also, color formers, or so-called couplers, remaining in the emulsion layers are subject to the action of UV to form undesirable color stains on the finished photographs. The fading and the discoloration of the color images are easily caused by UV of wavelengths near the visible region, namely, those of wavelengths from 300 to 400 nm. For the foregoing reasons, photographic elements typically incorporate a UV absorbing material in an upper layer.
By reference to "under", "above", "below", "upper", "lower" or the like terms in relation to layer structure of a photographic element, is meant the relative position in relation to light to when the element is exposed in a normal manner. "Above" or "upper" would mean closer to the light source when the element is exposed normally, while "below" or "lower" would mean further from the light source. Since a typical photographic element has the various layers coated on a support, "above" or "upper" would mean further from the support, while "below" or "under" would mean closer to the support.
Many types of UV absorbing materials have been described previously, and include those described in U.S. Pat. Nos. 3,215,530, 3,707,375, 3,705,805, 3,352,681, 3,278,448, 3,253,921, and 3,738,837, 4,045,229, 4,790,959. 4,853,471, 4,865,957, and 4,752,298, and United Kingdom Patent 1,338,265. Known UV absorbing materials often have many undesirable characteristics. For example, they tend to color and form stains due to their insufficient stability to UV, heat, and humidity. Also, a high-boiling organic solvent is usually required for the emulsification of the UV absorbing agents, which softens the layer and substantially deteriorates interlayer adhesion. In order to prevent these problems, a large amount of gelatin has been used in the layer containing the UV absorbent, resulting in a layer which may be unstable. Alternatively, a separate gelatin protective layer was provided over the UV absorbent containing layer. Such approach results in an undesirable thickening of the element. Furthermore, previously known UV absorbing agents, when provided in the uppermost layer of a photographic element, often migrate and crystallize at the surface of the layer. Thus, a gel overcoat would be used to minimize this undesirable blooming phenomenon. Furthermore, the droplets of such UV absorbing materials, when prepared by the conventional emulsification method described above, usually have particle sizes greater than 200 nm thereby producing light scattering with resulting deterioration of the element's photographic properties. The toxicity of such UV absorbing agents has also become an important issue recently.
It is known that polymer latexes obtained by polymerization of UV absorbing monomers, can be utilized as UV absorbing agents which do not have many of the disadvantages described above. At least three methods of adding polymeric UV absorbing agents in the form of latex to hydrophilic colloid composition, are known. The first method comprises adding a latex prepared by emulsion polymerization directly to a gelatin-containing silver halide emulsion. Emulsion polymerization is well known in the art and is described in F. A. Bovey, Emulsion Polymerization issued by Interscience Publishers Inc. New York, 1955. This is the most direct way of preparing a polymer latex.
A second method of forming a polymer latex is by solution polymerization of a monomeric mixture of UV absorbing monomer, a comonomer, and an ionic comonomer containing sulfonic, sulfuric, sulfinic, carboxylic or phosphoric acid, and their metal salts, such as acrylamido -2,2'-dimethyl-propane sulfonic acid, 2-sulfoethyl methacrylate, or sodium styrene sulfonate. The polymer solution obtained is then dispersed in aqueous solution and forms a latex.
The third method of forming a polymer latex is by solution polymerization of monomer mixture comprising UV absorbing monomer and hydrophobic comonomers. The polymeric UV absorber is isolated and dissolved in an auxiliary solvent, such as ethyl acetate, with a permanent solvent optionally also being present. This solution is mixed with an aqueous gelatin solution and the mixture passed through a high-shear device, such as a colloid mill, to form a latex.
Polymeric UV absorbing polymer latexes prepared by the foregoing type processes have been described in, for example, U.S. Pat. Nos. 3,761,272; 3,745,010; 4,307,184; 4,455,368; 4,464,462; 4,513,080; 4,340,664; GB 1,504,949; GB 1,504,950; British Patent 1,346,764; EP Application 0 190 003 and others. Some polymerlatexes containing polymers of certain specific structures, have been previously used in photographic elements. For example, U.S. Pat. No. 4,551,420 and U.S. Pat. No. 4,464,462 describe photographic elements with polymer latexes. U.S. Pat. No. 4,943,519 describes the use of latexes formed from various ultraviolet absorbing polymers in photographic film, the patent indicating that the latexes can be formed by emulsion or solution polymerization.
Among the three methods described above, emulsion polymerization is most desirable method of forming a polymer latex because no washing is required to remove solvents since no auxiliary solvent is used. Importantly, the size of the particles formed by emulsion polymerization is smaller than that produced by the other two methods. Smaller particles in the latex reduce light scattering in the photographic materials and improves dye density and gloss.
However, although UV absorbing polymer latexes prepared by emulsion polymerization have the advantages as described above, one of the disadvantages is that UV absorbing latexes prepared by emulsion polymerization exhibit poor light stability.
It would be desirable to provide a method of preparing photographic elements containing a absorbing polymer latex which is prepared by emulsion polymerization, but which nevertheless has good light stability. It would also be desirable to provide a photographic element containing a polymeric UV absorber composition, particularly one prepared by emulsion polymerization, such that the composition itself is relatively stable to light in the typical photographic element environment.